Ophiolites in New Zealand With Special Reference to Dun Mountain and Red Hill

Abstract

Ophiolites are mafic and ultramafic plutonic rocks and closely associated mafic volcanic rocks that are found in, or adjacent to, strongly deformed marine sediments. Alteration products of these rocks, such as serpentinite and amphibolite, are also considered to be ophiolites. Ophiolites generally occur as thin belts of serpentinite and mafic volcanics. These belts often extend for great distances and so form important geological reference lines, but in order to trace them beneath covering sediments it is necessary to combine geology and geophysics. Partially and completely serpentinised ultramafic rocks in ophiolite bodies at Dun Mountain and Red Hill have strong remanent magnetisation which is generally greater than the induced magnetisation. The directions of the remanent magnetisation are widely scattered so no quantitative analysis can be made of magnetic intensity measurements taken over these bodies. However, airborne magnetic intensity measurements can be used qualitatively to trace ophiolite belts. Gravity measurements are of little use in tracing serpentinites in New Zealand because serpentinite has a similar density (2.5-2.6 mg/m3) to the surrounding marine sediments. Unserpentinised ultramafic rocks have high density (3.3 Mg/m3) and the mafic intrusives and volcanics are moderately dense (2.9-3.0 mg/m3); gravity measurements can therefore be used to determine their shape and to trace them. By combining geology with gravity and magnetic intensity measurements the location and extent of several ophiolite belts in New Zealand have been determined. The longest (1200 km) and most important is the Dun Mountain Ophiolite Belt (DMOB) which crops out between Dâ Urville Island and Lumsden in the South Island, except where dextrally offset about 460 km by the Alpine Fault. A narrow, positive, linear magnetic anomaly, called the Dun Mountain Ophiolite Belt Magnetic Anomaly (DMOBMA), is associated with the DMOB and can be traced 730 km north from Dâ Urville Island under mainly Cenozoic strata in the western part of the North Island. In the North Island the DMOBMA is continuous, regular (100-200 nT, at 3 km altitude), and does not deviate more than 6 km from an arc with radius 430 km and centred at 38à °10â s and 170à °0â E On the opposite side of the Alpine Fault the DMOBMA is also well defined, and can be traced from the fault through Lumsden, to the coast near Inchclutha. The DMOB separates Hokonui Facies rocks (moderately deformed, shelf, Permian-Jurassic) to the west from Alpine Facies rocks (strongly deformed, flysch, Permian-Jurassic) to the east, and is thought to have been formed by obduction of oceanic crust at a lithospheric plate boundary. A second, positive; linear magnetic anomaly lies roughly parallel and 20-50 km west of the DMOB in the South Island, and consists of a broad (10-100 km wide) zone of many separate, ellipse shaped, magnetic "highs' (200-2000 nT, at 3 km altitude). The zone can be traced north from Nelson beneath Cenozoic sediments in Cook Strait, but cannot be recognised north of Manaia because of numerous magnetic anomalies associated with late Cenozoic volcanics. The zone is also offset 460 km by the Alpine Fault, and in the southern part of the South Island can be traced from the fault through the Darran and Takitimu Mountains to Foveaux Strait. The anomalies are caused by strongly magnetic rocks within late Paleozoic intrusives of the Rotoroa Igneous Complex (RIC) and its correlatives, and the zone is therefore called the Rotoroa Igneous Complex Magnetic Anomaly (RICMA). These two linear magnetic anomalies (DMOBMA, RICMA) comprise the Stokes Magnetic Anomaly and form one of the major geological reference lines in New Zealand. Another important ophiolite belt is the Haupiri Ophiolite Belt. Gravity and magnetic intensity measurements suggest that the belt extends from near Cape Farewell south to the Alpine Fault near Springs Junction. The belt reappears on the opposite side of the fault near Milford Sound and can be traced south-west to Charles Sound. A linear magnetic anomaly which starts about 40 km south-east of Charles Sound and extends 75 km south-west to the Last Cove Fault in Long Sound may represent a further continuation of this belt. The Dun Mountain and Haupiri Ophiolite Belts are important in determining past horizontal crustal movements in the New Zealand region. The present shapes of these belts and the Stokes Magnetic Anomaly indicate relative motion between the Australian and Pacific lithospheric plates. It is inferred that this motion began during the Oligocene, and caused bending of rocks in a zone between the plates. The Stokes Magnetic Anomaly was bent in a recurved arc, the ends of which were dextrally displaced at least 500 km, and the width of the anomaly was reduced to a minimum in the centre of the zone. With continued motion the rocks in the zone became ruptured along the Alpine Fault and the two parts were offset about 460 km. The total displacement of the Stokes Magnetic Anomaly across the New Zealand region is therefore about 1000 km.